Immobilized enzymes have gained increasing importance in both the preparative and the analytical fields, since the immobilized enzymes may be used several times and can be more readily separated from the reaction medium. Enzymes can be fixed to various carriers in different ways. Carriers to which the enzymes are bound via covalent bonds have proven to be particularly useful. Various organic polymers and inorganic materials have already been employed. Useful known carrier materials are polysaccharides (Sepharose, Sephadex, starch, etc.), synthetic polymers such as polyacrylamides, phenol-formaldehyde resins, and inorganic materials such as glass, aluminium oxides, and silicates.
A covalent binding of enzymes to a carrier in some cases is effected by a covalent bond to an amino group of the carrier through a bifunctional spacer to an amino group of the enzyme. In the event that the carrier material, due to its nature, does not contain free amino groups at its surface, these are first introduced by means of a chemical reaction. To this end, in the case of inorganic carriers, more specifically silicates, 3-aminopropyl trimethoxysilane (APTS), inter alia, has proven to be useful. The enzymes are coupled to these amino groups by convalent bond(s) via a bifunctional spacer. In addition, [3-(2-aminoethyl)aminopropyl]trimethoxysilane can be used.
In practice, among other substances, glutaraldehyde has proven to be particularly useful as a spacer, as it is capable of reacting with amino groups of the carrier as well as with amino groups of the enzyme.
A drawback inherent to organic carrier materials is the sensitivity thereof to organic solvents and higher temperatures and, in some cases, the high costs as well. In contrast to organic polymers, inorganic carrier materials such as glass, aluminates, and silicates, have only a relatively small reactive surface so that they can only hold a relatively small amount of enzyme and, hence, have to be used in substantially larger amounts.